1. Field of the Invention
This invention relates to a pathlength controller for a ring laser gyroscope and, more particularly, to an improved and simplified pathlength control assembly.
2. Description of Related Art
The Ring Laser Gyroscope has been developed as a logical replacement for the mechanical inertial gyroscope. Based upon the principles of the Sagnac Effect, ideally the ring laser gyroscope has minimal moving parts allowing extremely accurate rotational sensing. As originally envisioned, the ring laser gyroscope has at least two counter-propagating electromagnetic waves (such as light) which oscillate within an optical ring cavity. When the ideal ring laser gyroscope is stationary, no rotation is indicated by the sensor. As the ring cavity of the laser gyroscope is rotated about its central axis, the counter-propagating waves develop a beat frequency. Well above a characteristic lock-in zone a linear relationship between the beat frequency and the rotation rate of the gyroscope with respect to the inertial frame of reference may be established.
The working ring laser gyroscope requires various adjusting mechanisms to approach the ideal linear relationship between the beat frequency and the rotation rate of the gyroscope. Although the ideal ring laser gyroscope is characterized by a beat note proportional to the rotational rate, the two mode planar ring laser gyroscope requires rate biasing or mechanical dithering to prevent counter propagating waves from locking at low rotation rates. Mode locking is a major difficulty at low rotation rates where the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that of where lock-in occurs, and is then decreased, the frequency difference between the beams disappears at a certain input rotation. Lock-in arises from the coupling of light between the beams. An effective means of overcoming the lock-in effect of the counter-propagating modes of light within a two mode gyroscope is to mechanically dither the mirrors or body of the gyroscope.
Additionally, the optical pathlength of the gyroscope must be controlled and monitored to make certain that the resonant cavity operates at the center of the atomic spectra gain curve. Due to the multiplicity of their applications, ring laser gyroscopes are required to operate over a wide temperature range, such as -55.degree. C. to +75.degree. C. Since the laser light beam emitted by the active gain region of the gyroscope propagates around the ring laser by means of reflection off the surfaces of at least 3 mirrors, thermal expansion of the frame will cause a significant change in cavity resonant wavelength. It is therefore necessary to provide a pathlength control mechanism to slightly vary the optical pathlength of the gyroscope ring resonator in order to preserve the fundamental resonance of the cavity to which all sensing instrument components of the gyroscope are calibrated.
Even where low expansive glass materials are used for building a monolithic frame which supports the optical cavity path between the mirrors, the pathlength of a ring laser gyroscope will still experience a substantial change in path length during temperature changes. This change can be as much as 5 wavelength or more at the resonant frequency of the light produced by the gaseous active medium, such as a helium-neon mix. In an active path length control system, the changes in path length due to thermal expansions and contractions are monitored by detector electronics and provide feedback information for driving one or more piezoelectric transducers.
Another important function of the pathlength control assembly is to maintain the resonant frequency of the ring laser cavity at the peak or center of the inhomogeneous line of the gain medium. It is well known to persons skilled in the art that the dispersive effects caused by the departure of the resonant frequency from the peak of the gain curve result in the two counter propagating beams experience different indices of refraction, and consequently different optical pathlengths which leads to a false gyro output, i.e., bias. Furthermore, the bias, besides being temperature dependent, is highly erratic as its magnitude changes with the uncontrolled offset between the cavity resonant frequency and the gain line center.
With reference to prior art FIGS. 1A and 1B, preserving the optical pathlength means that the gain center line 12 of the atomic spectra resonant gain profile 10 should be centered with the curve at a point of maximum intensity. In order to preserve this setting during a variety of temperatures, piezo-electric transducer driven mirror assemblies 16 and 18 are electrically actuated to move axially as shown in FIG. 1A to compensate for changes in the pathlength of the ring laser optical path 20. Light intensity detectors 22 and 24 are positioned at either of the fixed mirrors 26 and 28 to sense rotation. Each of the transducer driven mirror assemblies 16 and 18 include piezoelectric actuators attached to a flexible annular surface or driver in order to accomplish the axial deflection of reflecting surfaces thereon.
Heretofore, various schemes for pathlength control assemblies such as assemblies 16 and 18 of FIG. 1A have been suggested. Among the designs for pathlength controllers are those included in U.S. Pat. No. 4,824,253 issued Apr. 25, 1989 to Butler (owned by the common assignee of this application); U.S. Pat. No. 4,861,161 issued Aug. 29, 1989 to Ljung; U.S. Pat. No. 4,691,323 issued Sep. 1, 1987 to Ljung; and UK Application GB 2,037,455 published Jul. 9, 1980, inventors Ljung and Williams. U.S. Pat. No. 4,861,161 to Ljung teaches a particular technique in the design of PLC assembly to minimize the mirror tilt.
In particular, attention is drawn to FIG. 2 as an example of the complex structure of a PRIOR ART pathlength control assembly. A path length controller mirror assembly 30 is shown generally to include a membrane-type mirror housing 38 which supports the mirror 44, which mirror is positioned facing into the gyroscope frame 40 for reflecting light within the optical path off of its surface. The housing 38 has an outer cylinder 34 and a central mirror post 36. The annular surface 48 of the mirror post 36 and the outer annular surface 52 of the housing 38 are flush against the circular backing plate 32. The circular backing plate 32 is sandwiched between the mirror housing 38 and the driver body 50. The driver body includes a driver post 54, which during activation of the cavity length control assembly causes the driver post surface 56 to move axially along the direction shown at 42. Such axial movement of the driver post 54 causes axial movement by the mirror post 36 against the flexible mirror membrane 46, thereby allowing axial movement of the mirror 44 between a rest position and a flexural position 44' (shown in phantom). The driver body 50 has an outer surface 58 which is flush against the backside of the backing plate 32.
Positioned on either side of the back end of the driver body 50 are two piezo-electric elements 60 and 62. These elements are often bimetallic or bimorphic, such that when they are alternatively polarized by applying a voltage thereto through electrical terminals 64 and 66, the driver body 50 and driver post 54 move axially along the central axis of the driver body 50, back and forth as needed in the axial direction 42. Such movement results in positioning the backing plate to a new position 68, and moving the diaphragm membrane 46 of the mirror housing 38 to a new position 72, all of which results in the desired axial movement 42 of the mirror surface 44 out to 44'. It will be noted that in the prior art FIG. 2, a relatively complex structure, including a separate driver body 50, was needed to achieve a proper and balanced pathlength control assembly. Such a structural complexity was dictated by the need to drive the mirror 44 in one axial direction 42 , while the driver 50 would not act to destabilize the dynamic symmetry of the assembly 30. Certainly, the more complex that a pathlength controller is, the more costly becomes the gyroscope design and fabrication.